Fuel-burning appliances are of many types. Fuels used include gas, kerosene, solid fuel and many others. Fuel-burning appliances are commonly used for e.g. heating in both domestic and commercial applications. A well-known and dangerous by-product of such appliances, however, is carbon monoxide. Carbon monoxide is an odorless, tasteless, poisonous gas, which cannot readily be detected. In conjunction with fuel-burning appliances, therefore, some form of CO detector is frequently supplied, either as a separate detector or as an integral part, of the appliance.
One type of device used to sense the presence of CO is an Oxygen Depletion Sensor (ODS), which responds to a drop in the oxygen concentration of the surrounding air. ODS sensors commonly produce an alarm response when the oxygen level drops to, say, 95% of the normal level. A typical alarm response might be, for example, the cutting off of the fuel supply and/or the sounding of an alarm signal. It is well known, however, that concentrations of CO which are not low enough to trigger an ODS may still be high enough to cause ill effects, particularly over a long period. It has therefore been suggested to use a detector which reacts to the presence of carbon monoxide (as opposed to the absence of oxygen), either instead of or as well as an ODS.
The provision of both a carbon monoxide sensor and an ODS in a fuel-burning appliance has hitherto required the provision of two separate circuits in the appliance, bringing with it increased complexity and costs. The absence of both types of sensor, however, can lead to ill health and possibly fatal effects.